A rock bolt is a long anchor bolt used to reinforce rock cuts, cavities and walls of tunnels. A rock bolt installed in its place can also be used for suspension of various structures. Typically in reinforcing a rock, rock bolts are used as an array of several rock bolts installed at a distance from each other. Rock bolts function in such a way as to bind the rock mass present in the vicinity of the surface of the rock, which may include openings and boulders, to the firmer and more solid internal part of the rock and this way to prevent rockfall.
Installation of a rock bolt is carried out in such a way as to drill a long closed hole into the wall of the rock to be reinforced for each rock bolt. The rock bolt is inserted into the closed hole for a desired depth. The rock bolt is secured in its place by expanding an expansion member present at the inner end, i.e. the tip, of the rock bolt so that the expansion member becomes pressed against the wall of the borehole. Finally, a binder, such as concrete or resin, is injected into the borehole around the rock bolt for cementing the rock bolt.
Rock bolts are known from the prior art, for example from publications EP 1546508 B1, U.S. Pat. Nos. 5,636,945, 4,655,644, 4,764,055, 4,848,971 and 4,904,123.
In this description the term “inner” means that the part is located deeper in the borehole within the rock than another part characterized by the term “outer”.
A rock bolt includes a drawbar having an inner end to be inserted into the borehole and an outer end extending out from the borehole. Further, the rock bolt includes tightening members provided at the outer end of the drawbar for drawing the drawbar outward relative to the borehole in order to tighten the rock bolt in its place. Further, the rock bolt includes a first cone member substantially immovably connected to the drawbar in the vicinity of the inner end, and a first cone surface. In addition, the rock bolt includes an expansion member including a first end which is the end on the side of the inner end of the drawbar, a second end which is the end on the side of the outer end of the drawbar, and a first cone abutment surface. The first cone abutment surface is located in the interior of the expansion member in the vicinity of the first end thereof. The first cone member is provided in the interior of the first end of the expansion member in such a way that the first cone surface abuts against the first cone abutment surface. The cone member functions as a wedge which expands the expansion member as it penetrates deeper therein. The expansion member is adapted to expand as the drawbar is drawn outward by means of the tightening members and the first cone member is simultaneously moved relative to the expansion member for expanding the expansion member and engaging the expansion member to the wall of the borehole. The expansion member includes an internal second cone abutment surface located in the interior of the expansion member in the vicinity of the second end thereof. The rock bolt further includes a second cone member at least partially fitted in the interior of the second end of the expansion member. The second cone member includes a through hole, through which the drawbar is adapted to extend with a clearance, and a second cone surface tapering inward relative to the expansion member and abutting against the second cone abutment surface of the expansion member. In addition, the rock bolt includes an elongated pushing member having a first end bearing on the tightening members and a second end bearing against the second cone member on the opposite side relative to the expansion member so that, as the drawbar is tightened by the tightening members, the pushing member simultaneously pushes the second cone member to the interior of the expansion member for expanding the expansion member and engaging the expansion member to the wall of the borehole. The second pushing member is a tube fitted around the drawbar.
The two cone members provide expansion of the expansion member simultaneously at both ends, thereby providing safety-increasing immediate locking in place instantly at the beginning of tightening. The final tightening can be carried out without hurry. The tightening is easy because it can be effected with a low torque.
Regarding the rock bolts provided with the biconical wedge and expansion anchoring mechanism according to the prior art described above, reference is also made to publications JP 5601811U and DE 3105038 A1. A problem therein is that the use of a binder to be injected is not possible.